In recent years, ceramic-metal composites have been widely used as circuit boards such as substrates for power transistor module substrates and substrates for switching power supply modules. The ceramic-metal composites include ceramic substrates and metal plates, such as copper plates, bonded to the substrates. Commonly, examples of the ceramic substrates include aluminum nitride substrates and silicon nitride substrates which have electrically insulating property and a high thermal conductivity.
A conventional ceramic-metal composite such as a ceramic circuit board for power modules is manufactured in such a manner that a ceramic substrate made of aluminum nitride (AlN), which has high thermal conductivity, or silicon nitride (Si3N4), which has high strength and thermal conductivity, is bonded to a metal plate, such as a copper plate, having high thermal conductivity by a process such as a refractory metal process using Mo or W, a DBC process making use of the eutectic reaction between copper and oxygen, or an active metal process and a metal portion is then patterned through an etching treatment.
In particular, a ceramic-metal composite (ceramic circuit board) 11 having a circuit formed by etching a copper plate as described above includes a ceramic substrate 12, a metal circuit plate 13 which is a copper plate and which is bonded to a surface of the ceramic substrate 12, and a backing metal plate 14 which is a copper plate and which is bonded to another surface of the ceramic substrate 12 as shown in FIGS. 1 to 3. Examples of a process for bonding a metal plate to the ceramic substrate 12 include such a direct bonding process, refractory metal metallizing process, and active metal process as described below.
The direct bonding process is a so-called direct bonding copper process (DBC process) in which a copper plate is directly bonded to the ceramic substrate 2 using a eutectic liquid phase such as a Cu—Cu2O phase. The refractory metal metallizing process is a process in which a refractory metal such as Mo or W is fixed on the ceramic substrate by firing. Further, the active metal process is a process in which a metal plate is bonded to the ceramic substrate 12 through a brazing alloy layer 15 disposed therebetween. The active metal process is widely and generally used in such a way that a brazing paste, which is obtained by adding an active metal such as Ti to a eutectic brazing alloy with a Cu—Ag eutectic composition (72% Ag and 28% Cu on a weight basis), is applied between a ceramic substrate and a metal member and then heat-treated at an appropriate temperature such that the ceramic substrate and the metal member are bonded to each other, because high strength and high sealing performance can be achieved.
An example of a bonding structure prepared by bonding the ceramic substrate and the metal member together is a structure including a metal circuit plate and a brazing alloy layer extending over a portion of a side surface of the metal circuit plate as disclosed in Patent Document 1. This bonding structure is effective in achieving high bonding strength and therefore is capable of achieving high reliability against thermal cycles (heat cycles).
Patent Document 2 discloses another bonding structure in which the area of the interface between a metal circuit plate and a ceramic substrate is less than the area of the front surface thereof and the area of a brazing alloy layer containing an active metal is greater than the area of a bonding surface of the metal circuit plate. According to this bonding structure, a bonding surface of the metal circuit plate is fixed on the brazing alloy layer but the front surface thereof is not particularly restricted and is therefore expandable depending on the difference in coefficient of linear expansion therebetween. This allows the residual stress to be reduced, thereby enhancing the heat cycle resistance.
Examples of a known process for forming a circuit include a process using a copper plate patterned by pressing work or etching treatment, a process for patterning a bonded metal portion by etching or another technique, and a similar process. A ceramic circuit board prepared by the DBC process or an active metal brazing process has a simple structure, a small heat resistance, and an advantage in that the ceramic circuit board can cope with high-current (high-power use) or high-integration semiconductor chips.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-347469    Patent Document 2: Japanese Unexamined Patent Application Publication No. 6-263554
In recent years, high-integration semiconductor elements and high-power semiconductor devices including ceramic-metal composites used as circuit boards have rapidly spread. Thermal stresses or thermal loads repeatedly applied to the ceramic-metal composites are likely to be increasing; hence, the ceramic-metal composites need to have sufficient bonding strength and resistance to the thermal stresses and thermal cycles. The ceramic-metal composites need to have high definition circuit patterns (extremely fine circuit patterns) because such high-integration semiconductor elements are mounted on the ceramic-metal composites. The ceramic-metal composites, which may be used as ceramic circuit boards, need to have bonding portions that have high bondability (voidless bonding) so as not to contain any voids and further need to have high heat durability and high heat cycle resistance.
However, conventional ceramic-metal bonding techniques have a disadvantage in that bondability varies significantly due to changes in bonding atmospheres during large-scale manufacture (mass-production) or depending on the lot of substrates. Even if high bondability is achieved, requirements for heat cycle resistance have not been sufficiently satisfied. Therefore, there has been posed a problem in that semiconductor devices including ceramic-metal composites used as circuit boards have low reliability or are low in production yield.
Furthermore, there has been posed also a problem in that conventional ceramic-metal composites are liable to lower an operating reliability because molten brazing alloys excessively spread widely over surfaces of the conventional ceramic-metal composites thereby to cause short circuit accidents between adjacent metal circuit plates.